Asymmetrical switching occurs when the two switches in a half-bridge power converter are turned on and off complementarily. In conventional switch-mode converters, the power switches are typically maintained in a conducting state for an equal duration during each half of the switching period. For one duty cycle, alternately one switch, then the other switch, conducts during successive half periods of the duty cycle for the same length of time with a corresponding dead time between each conduction time period. Asymmetrical half-bridge converters differ from the conventional switch-mode converters in that the switches conduct for unequal lengths of time with only a small deadtime between conduction periods.
An analysis of a zero-voltage-switching (ZVS) asymmetrical half-bridge converter with pulse-width-modulation (PWM) control is described, for example, in a paper entitled "Static and Dynamic Analysis of Zero-Voltage-Switched Half-Bridge Converter with PWM Control" by Tamotsu Ninomiya, et al., Proceedings of IEEE PESC '91, pp. 230-237 (1992); see also "Static and Dynamic Analysis of ZVS-PWM Half Bridge Converter" by T. Ninomiya, et al., IEE of Japan, MAG-90-82, August 1990, both of which are incorporated herein by reference. Ninomiya, et al., analyses a half-bridge converter with an asymmetrical PWM control scheme and demonstrates quantitatively the improvement of the control characteristics performed by the asymmetrical regulation of a pair of switches employed therein. Furthermore, Ninomiya, et al., teaches that the small deadtime between conducting periods in an asymmetrical converter circuit allows lossless commutation of the power switches.
Another asymmetrical half-bridge ZVS converter is described in, for example, U.S. Pat. No. 5,245,520, issued on Oct. 10, 1993, to Imbertson, entitled "Asymmetrical Duty Cycle Power Converter," which is incorporated herein by reference. Imbertson omits the resonant inductor described in Ninomiya's converter by utilizing the energy stored in the leakage inductance (or an auxiliary inductor) and the magnetizing inductance to reach ZVS. This practice is commonly used in designing ZVS full-bridge DC/DC converters and clamping-mode forward converters. This arrangement permits ZVS without introducing additional voltage and current stresses on the switches and the converter is suitable for high frequency applications.
An asymmetrical half-bridge converter, however, suffers from having a DC bias current in the converter's isolation transformer and a large ripple current component in the output of the converter. The output ripple current can be reduced or eliminated at the expense of increasing the DC bias current in the converter's transformer. The DC bias current necessitates that magnetic core of the isolation transformer be increased to prevent the transformer from saturating. Therefore, care must be taken when determining the size of the transformer to take into account the largest DC bias current that may be present. This inevitably results in an increase in the overall size, weight and cost of the converter.
Accordingly, what is needed in the art is an improved converter that mitigates the above-identified problems and, more particularly, there is a need for an improved converter wherein the isolation transformer is not burdened with all of the DC bias current generated in the converter.